There are two issues to the rate one being absolute cost and the other being diminishing returns.
On the absolute cost, it is minimal at 20% given the low amount of savings but wouldn't be minimal at 1000% (of course I know that was hyperbole).
In terms of diminishing returns, 20% is a widely available "high/promotional" rate on stablecoins right now. UST is paying it and a major new one, USN, is rumored to be planning the same soon. Anything much below that falls into the category of why should I bother with Hive and anything much above that is a waste of money since anyone seeking yield would find 20% enough. A case could be made for a slightly higher rate, say 22% or 25% to account for the fact that Hive is tiny and obscure. Still going too far is a waste of money when you're already offering something competitively attractive.
"Sustainable" (nonpromotional; derived from demand for loans) DeFi stablecoin rates are probably more like 10% or a bit less. We're vaguely competitive at 12% already but perhaps not enough to offset the fact that Hive is more obscure and more difficult to access. Again, some premium makes sense to attract usage and attention, but going too far is a waste of money.